Capping layer for EUV optical elements

ABSTRACT

Optical elements such as multilayered EUV mirrors are provided with protective capping layers of diamond-like carbon (C), boron nitride (BN), boron carbide (B 4 C), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), B, Pd, Ru, Rh, Au, MgF 2 , LiF, C 2 F 4  and TiN and compounds and alloys thereof. The final period of a multilayer coating may also be modified to provide improved protective characteristics.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to capping layers for optical elements,e.g. multilayer mirrors, for use with extreme ultraviolet (EUV)radiation. More particularly, the invention relates to the use ofcapping layers on optical elements in lithographic projection apparatuscomprising:

an illumination system for supplying a projection beam of radiation;

a first object table provided with a mask holder for holding a mask;

a second object table provided with a substrate holder for holding asubstrate; and

a projection system for imaging an irradiated portion of the mask onto atarget portion of the substrate.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, catadioptric systems,and charged particle optics, for example. The illumination system mayalso include elements operating according to any of these principles fordirecting, shaping or controlling the projection beam, and such elementsmay also be referred to below, collectively or singularly, as a “lens”.In addition, the first and second object tables may be referred to asthe “mask-table” and the “substrate table”, respectively.

In the present document, the invention is described using a referencesystem of orthogonal X, Y and Z directions and rotation about an axisparallel to the I direction is denoted Ri. Further, unless the contextotherwise requires, the term “vertical” (Z) used herein is intended torefer to the direction normal to the substrate or mask surface orparallel to the optical axis of an optical system, rather than implyingany particular orientation of the apparatus. Similarly, the term“horizontal” refers to a direction parallel to the substrate or masksurface or perpendicular to the optical axis, and thus normal to the“vertical” direction.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the mask(reticle) may contain a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto an exposure area(die) on a substrate (silicon wafer) which has been coated with a layerof photosensitive material (resist). In general, a single wafer willcontain a whole network of adjacent dies which are successivelyirradiated via the reticle, one at a time. In one type of lithographicprojection apparatus, each die is irradiated by exposing the entire areticle pattern onto the die at ones; such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus—which iscommonly referred to as a step-and-scan apparatus—each die is irradiatedby progressively scanning the reticle pattern under the projection beamin a given reference direction (the “scanning” direction) whilesynchronously scanning the wafer table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M generally <1), the speed V at which the wafertable is scanned will be a factor M times that at which the reticletable is scanned. More information with regard to lithographic devicesas here described can be gleaned from International Patent ApplicationWO97/33205, for example.

Until very recently, lithographic apparatus contained a single masktable and a single substrate table. However, machines are now becomingavailable in which there are at least two independently moveablesubstrate tables; see, for example, the multi-stage apparatus describedin International Patent Applications WO98/28665 and WO98/40791. Thebasic operating principle behind such multi-stage apparatus is that,while a first substrate table is at the exposure position underneath theprojection system for exposure of a first substrate located on thattable, a second substrate table can run to a loading position, dischargea previously exposed substrate, pick up a new substrate, perform someinitial measurements on the new substrate and then stand ready totransfer the new substrate to the exposure position underneath theprojection system as soon as exposure of the first substrate iscompleted; the cycle then repeats. In this manner it is possible toincrease substantially the machine throughput, which in turn improvesthe cost of ownership of the machine. It should be understood that thesame principle could be used with just one substrate table which ismoved between exposure and measurement positions.

In a lithographic apparatus the size of features that can be imaged ontothe wafer is limited by the wavelength of the projection radiation. Toproduce integrated circuits with a higher density of devices, and hencehigher operating speeds, it is desirable to be able to image smallerfeatures. While most current lithographic projection apparatus employultraviolet light generated by mercury lamps or excimer lasers, it hasbeen proposed to use shorter wavelength radiation of around 13 nm. Suchradiation is termed extreme ultraviolet (EUV) or soft x-ray and possiblesources include laser plasma sources or synchrotron radiation fromelectron storage rings. An outline design of a lithographic projectionapparatus using synchrotron radiation is described in “Synchrotronradiation sources and condensers for projection x-ray lithography”, J BMurphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).

Optical elements for use in the EUV spectral region, e.g. multilayeredthin film reflectors, are especialy sensitive to physical and chemicaldamage which can significantly reduce their reflectivity and opticalquality. Reflectivities at these wavelengths are already low compared toreflectors at longer wavelengths which is a particular problem since atypical EUV lithographic system may have nine mirrors; two in theillumination optics, six in the imaging optics plus the reflectingreticle. It is therefore evident that even a “small” decrease of 1-2% inthe peak reflectivity of a single mirror will cause a significant lightthroughput reduction in the optical system.

A further problem is that some sources of EUV radiation, e.g. plasmabased sources, are “dirty” in that they also emit significant quantitiesof fast ions and other particles which can damage otical elements in theillumination system.

Proposals to reduce these problems have involved maintaining the opticalsystems at very high vacuum, with particularly stringent requirements onthe partial pressures of hydrocarbons which may be adsorbed onto theoptical elements and then cracked by the EUV radiation to leave opaquecarbon films.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide optical elements,including multilayer mirrors, for use in lithographic projectionapparatus using extreme ultraviolet radiation (EUV) for the projectionbeam, that are more resistant to chemical and physical attack.

According to the present invention, this and other objects are achievedin a lithographic projection apparatus comprising:

an illumination system for supplying a projection beam of radiation;

a first object table provided with a mask holder for holding a mask;

a second object table provided with a substrate holder for holding asubstrate; and

a projection system for imaging an irradiated portion of the mask onto atarget portion of the substrate; characterised by:

at least one optical element having a surface on which radiation of thesame wavelength as the wavelength of said projection beam is incidentand a capping layer covering said surface, said capping layer beingformed of a relatively inert material.

The optical element may be a beam modifying element such as a reflector,e.g. a multilayer near-normal incidence mirror or a grazing incidencemirror, included in one of the illumination and projection systems: anintegrator, such as a scattering plate: the mask itself, especially if amultilayer mask; or any other optical element involved in directing,focussing, shaping, controlling, etc. the projection beam. The opticalelement may also be a sensor such as an image sensor or a spot sensor;

The relatively inert material in particular should be resistant tooxidation and may be selected from the group comprising: diamond-likecarbon (C), boron nitride (BN), boron carbide (B₄C), silicon nitride(Si₃N₄), silicon carbide (SiC), B, Pd, Ru, Rh, Au, MgF₂, LiF, C₂F₄ andTiN and compounds and alloys thereof.

The capping layer should have a sufficient thickness to protect theunderlying optical element from attack, so that the capping layer iseffecively “chemically opaque”, yet not be too thick so as to absorb toomuch of the incident radiation. To these ends, the capping layer mayhave a thickness in the range of from 0.5 to 10 nm, preferably from 0.5to 6 nm and most preferably from 0.5 to 3 nm.

The capping layer may itself have a multi-layer structure, e.g. of twolayers, with the outermost layer chosen both for improved chemicalresistance and low refractive index at the wavelength of the projectionbeam to improve reflectivity or transmissivity.

A second aspect of the invention provides a device manufacturing methodusing a lithographic apparatus comprising

an illumination system for supplying a projection beam of radiation;

a first object table provided with a first object holder for holding amask;

a second object table provided with a second object holder for holding asubstrate; and

a projection system for imaging an irradiated portion of the mask onto atarget portion of the substrate; said method comprising the steps of:

providing a mask containing a pattern to said first object table;

providing a substrate at least partially covered by a layer ofenergy-sensitive material to said second object table;

irradiating said mask and imaging irradiated portions of said patternonto said substrate; characterised in that:

said lithographic projection apparatus comprises at least one opticalelement having a surface on which radiation of the same wavelength asthe wavelength of said projection beam is incident and a capping layercovering said surface, said capping layer being formed of a relativelyinert material.

In a manufacturing process using a lithographic projection apparatusaccording to the invention a pattern in a mask is imaged onto asubstrate which is at least partially covered by a layer ofenergy-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake WEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping) metallization,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-0672504.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetarea”, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its attendant advantages will be describedbelow with reference to exemplary embodiments and the accompanyingschematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to theinvention;

FIG. 2 is a graph of layer thicknesses in a 51 period optimised Mo/Sistack according to the invention;

FIG. 3 is a graph of layer thicknesses in a 50 period Mo—Ru/Si stackaccording to the invention;

FIG. 4 is a graph of R⁹ vs. wavelength in the 13.4 nm region for variousmirrors embodying the invention and a conventional mirror forcomparison;

FIG. 5 is a graph of layer thicknesses in a 50 period Mo—Ru—Sr/Si stackaccording to the invention;

FIG. 6 is a graph of layer thicknesses in a needle optimised 50 periodMo—Ru—Sr/Si stack according to the invention;

FIG. 7 is a graph of layer thicknesses in an 80 period Ru—Sr/Be stackaccording to the invention;

FIG. 8 is a graph of R⁹ vs. wavelength in the 11.3 nm region for variousmirrors embodying the invention and a conventional mirror forcomparison;

FIG. 9. Is a graph showing R⁹ vs. wavelength for various stacks, bothconventional and according to the invention as well as the emissionintensity of a Xe-jet laser-induced plasma source;

FIG. 10 is a graph showing R and R⁹ vs. wavelength for a Rh—Ru/Sr—Cestack according to the invention;

FIG. 11 is a graph of layer thicknesses in an optimised Rh—Ru/Sr—Cestack according to the invention;

FIG. 12 is a graph showing R versus wavelength for a Rh—Ru/SiO₂-aerostack according to the invention; and

FIG. 13 is a diagram of a multilayer coating having a capping layeraccording to the invention.

In the various drawings, like parts are indicated by like references.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

a radiation system LA, IL for supplying a projection beam PB of EUVradiation;

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to first positioningmeans PM for accurately positioning the mask with respect to item PL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to second positioning means PW for accurately positioningthe substrate with respect to item PL;

a projection system (“lens”) PL (e.g. a refractive or catadioptricsystem or a reflective system) for imaging an irradiated portion of themask MA onto a target portion C (die) of the substrate W.

The radiation system comprises a source LA (e.g. an undulator or wigglerprovided around the path of an electron beam in a storage ring orsynchrotron or a laser-induced plasma source) which produces a beam ofradiation. This beam is passed along various optical components includedin illumination system (“lens”) IL so that the resultant beam PB iscollected in such a way as to give uniform illumination at the entrancepupil and the mask.

The beam PB subsequently impinges upon the mask MA which is held in amask holder on a mask table MT. Having been selectively reflected by themask MA, the beam PB passes through the lens PL, which focuses the beamPB onto a target area C of the substrate W. With the aid of firstpositioning means PW and the interferometric displacement measuringmeans IF, the substrate table WT can be moved accurately, e.g. so as toposition different target areas C in the path of the beam PB. Similarly,the positioning means PM can be used to accurately position the mask MAwith respect to the path of the beam PB, e.g. after mechanical retrievalof the mask MA from a mask library. The references M1, M2 correspond toreticle alignment marks and the references P1 and P2 correspond to waferalignment marks. In general, movement of the object tables MT, WT willbe realized with the aid of a long stroke module (coarse positioning)and a short stroke module (fine positioning), which are not explicitlydepicted in FIG. 1.

The depicted apparatus can be used in two different modes:

In step mode, the mask table MT is kept essentially stationary, and anentire mask image is projected at ones (i.e. a single “flash”) onto atarget area C. The substrate table WT is then shifted in the x and/or ydirections so that a different target area C can be irradiated by thebeam PB;

In scan mode, essentially the same scenario applies, except that a giventarget area C is not exposed in a single “flash”. Instead, the masktable MT is movable in a given direction (the so-called “scandirection”, e.g. the x direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=¼ or ⅕). In this manner, a relatively large target area C can beexposed, without having to compromise on resolution.

The illumination system IL may be constructed as described in copendingEuropean Patent Application 00300784.6 (applicant's ref P-0129), whichis hereby incorporated by reference.

EXAMPLES

The examples of the invention described below are obtained fromcomputations performed using the thin film design program TFCalc(Software Spectra Inc.) and verified using LPro (4D Technology Ltd.).The built-in global and needle optimisation routines of TFCalc were usedfor the optimisation process, as described in A. V. Tikhonravov, Appl.Opt. 32, 5417 (1993), A. V. Tikhonravov, M. K. Trubetskov and G M.DeBell, Appl. Opt. 35, 5493 (1996) and J. A. Dobrowski and R. A. Kemp,Appl. Opt. 29, 2876 (1990), which references are incorporated herein byreference. The optical constants of the various materials, namely thecomplex refractive index N=n−ik are derived from atomic scatteringfactors by Henke et. al. and were obtained from the CXRO web server atBerkeley (B. L. Henke, E. M. Gullikson, and J. C. Davis, Atomic Data andNuclear Data Tables, 54(2), 181-342 (1993);http://www.cxro.lbl.gov/optical_constants/). The values of n and k forthe materials used were downloaded as functions of wavelength from 6 nmto 42 nm and as such the wavelength dependence of n and k is implicit inall calculations. The values of n and k for various materials at somewavelengths of particular interest are tabulated in Table 1 below. Todemonstrate the performance enhancement of the reflectors according tothe invention, we assume ideal “white” light illumination in theexamples below.

COMPARATIVE EXAMPLE 1

Comparative Example 1 is a standard Si-based multilayer stack comprisingan unoptimised 50-period Mo/Si system grown on a Zerodur (RTM) glasssubstrate, with a partition ratio Γ=0.4, yielding d_(Mo)=2.8 nm andd_(Si)=4.1 nm. In addition, it is assumed that the final Si layer willundergo oxidation and effectively form a ^(˜)2 nm layer of native oxide.Analysis of such a stack yields a peak reflectivity at ^(˜)13.4 nm ofR=0.731. This stack provides the reference for performance comparisonsof stacks according to the invention.

EXAMPLES 2 to 23

Examples 2 to 23 according to the invention consist of variations on thestack of reference example 1 as detailed in Table 2 below. In Table 2,column 2 gives the materials used in the layers of the stack; column 3gives the optimization applied: N indicates none, Y indicates globaloptimization and Y(n) indicates needle optimization (described furtherbelow); column 4 gives the capping layer applied; column 5 gives thepeak reflectivity R; column 6 gives the R⁹peak reflectivity in relativeunits and column 7 gives the R⁹int (integrated) reflectivity in relativeunits.

For a 9-reflector system, a more useful measure of optical throughput isthe value of R⁹, which the net reflectivity of a series of ninereflectors. R⁹int is the area under the curve in the R⁹ vs. λ(wavelength) spectrum. The variation between R⁹peak and R⁹int for agiven stack is an indication of the variation in the spectral half-widthwhich is a function of the optimization process, or the incorporatedmaterials, or the capping layer material, or any combination of thethree.

The final surface layer of all of examples 2 to 20 is a 4.1-4.5 nm Silayer on which the capping layer specified in column 4 is deposited, orgrown in the case of SiO. Growing the SiO₂ consumes the surface Si layerso that in the case of Example 2 the top two layers are 2 nm of Si, theremains of the approximately 4 nm Si layer prior to oxidation and whichmay be regarded as the final layer of the multilayer, and 2 nm SiO₂.Examples 21 to 23 are terminated with a 4.0 to 4.4 nm Rb layer uponwhich the capping layer specified in column 4 is deposited.

Example 2 is an unoptimized Mo/Si stack in which a 2 nm native oxide isallowed to grow on a 6 nm Si top layer (compared to the 4 nm top layerof comparative example 1), resulting in a 1% increase in R, a 13%increase in R⁹peak and a 7% increase in R⁹int.

In example 3, a 25% gain in R⁹int is achieved by deposition of a 2 nm Bcapping layer. Further increases in examples 4 to 7 follow by selectingRh or Ru as capping layers and optimising the stack. A gain of up to 36%for a two-component (Mo/Si) multilayer stack can be achieved byoptimization, as shown by example 7.

FIG. 2 shows the layer structure of a 51 period (102 layer) optimizedMo/Si stack with a 1.5 nm capping layer. In the Figure, layer 0 is thesubstrate surface. As can be seen, the optimisation of the Mo/Si stackresults in a gradual, smooth variation of the layer thicknesses throughthe stack while the period width remains nominally constant at about 6.8to 7.0 nm. Near the substrate, d_(Mo)≈d_(Si)≈3.5 nm varying tod_(Mo)=2.7 nm and d_(Si)≈4.2nm near the surface. In the stackillustrated in FIG. 2 the partition ratio Γ remains at about 0.4 for thefirst 20 periods from the surface (one period=one pair of layers, i.e.one Mo layer and one Si layer) and thereafter gradually changes to about0.5 at the substrate. Thus it appears that the higher the absorption inthe material, the lower the thickness near the surface, for an optimumreflectivity response. This phenomenon is discussed further below.

The three component system of examples 8 to 12 is set up initially as atwo-component Mo/Si stack with the third material interleaved betweenthe Mo and Si layers with its initial thickness set to zero. The globaloptimization process then varies the thicknesses of all the layers untila pre-set reflectivity target is approached. In the case of Mo—Rh/Si andMo—Ru/Si, Mo is favored near the surface and Rh or Ru near the substratewhereas, in the Mo—RbCl/Si system, RbCl (which is a single entity)partially substitutes for Si in the centre of the stack, i.e. the sum ofthe thicknesses of the adjacent RbCl and Si layers approaches thethickness of Si in a standard stack. The layer structure for theMo—Ru/Si stack is shown in FIG. 3. This stack has 50 Si layers,including the uppermost layer, and therefore has 148 layers in total,plus a 1.5 nm Ru capping layer. In the figure, layer 0 is the substratesurface. A 50% gain in computed throughput is observed for the Mo—Ru/Sisystem over the standard Mo/Si stack.

Example 12 shows a further improvement in R⁹int for the Mo—Ru/Si systemusing needle optimization. In the needle optimization routine,additional layers of designated materials, in this case, Mo, Ru and Rh,with vanishingly small thicknesses, are periodically added to the stack.These layers are then allowed to grow or be rejected by a localoptimization process. The needle-optimized stack therefore also containsRh and additional layers of Mo, the net result of which is a 59%increase in R⁹int compared to the standard stack. It is also worthnoting that in this case R⁹int>R⁹peak with the peak reflectivity of0.764 only marginally lower than for the standard optimized Mo—Ru/Sistack. This indicates that a substantially greater spectral half-widthresults from the needle optimization process as can be seen in FIG. 4,which is a graph showing R⁹ vs. wavelength in the 13.4 nm region. Line Ais for the standard Mo/Si stack, reference example 1; B is optimizedMo/Si, example 4; C is Mo—Ru/Si needle optimized, example 12; D isMo—Ru—Sr/Si needle optimized, example 19, and E is Mo/Rb optimized,example 22.

The order of layers in the three component stacks may be varied. Forexample, Rh—Mo/Si may be used instead of Mo—Rh/Si and Ru—Mo/Si insteadof Mo—Ru/Si

The four-component stacks, examples 13 to 20, were built in a similarmanner to the three component stacks described above. The mostfavourable combination is Mo—Ru—Sr/Si with up to an 88% relativeincrease in output intensity. FIG. 5 shows the layer thicknesses (nm) ofa 50 period Mo—Ru—Sr/Si stack with a Ru capping layer. As before, layer0 indicates the substrate surface. Again, within the first 50 layersfrom the substrate Ru predominates over Mo. The spikes in the Mo layerthickness profile indicate layers where the Ru layer has been whollyreplaced by Mo as suggested by the numerical optimization technique.This is not essential to the gain in R⁹int and the relevant Mo layerscan be replaced by pairs of Mo and Ru layers. Sr performs a similarfunction to Si in the stack as it has a high value of n and a lowextinction coefficient, k, (see Table 1). The low absorption within theSr layers makes it preferable in the top half of the stack. As with theMo—Ru/Si example discussed above, the sums of the thicknesses of Si andSr and Ru and Mo approximate respectively to the optimised Si and Mothicknesses shown in FIG. 2. The preferred order of the elements is:Ru—Mo—Sr—Si. The grouping of layers may also be varied, e.g. Ru—Mo—Sr/Simay be regarded as Ru—Mo/Sr—Si for calculation purposes.

FIG. 6 shows the layer thicknesses of a needle-optimized 50 period (50Si layers) Mo—Ru—Sr/Si stack. Rh is included only in the lower half ofthe stack and predominantly in the first 40 layers. In the lowest layersRh is preferred over Ru because of its higher optical contrast with Si,in spite of its higher extinction coefficient.

Sr and Y are less easily depositable owing to the complex chemistry of Yand the high reactivity of Sr, so are less preferred, but still showadvantages over the conventional stack. Mo—Ru—Zr/Si and Mo—Ru—RbCl/Sishow particular promise, as do the same layers in the order Ru—Mo—Zr/Siand Ru—Mo—RbCl/Si.

A comparison of the optical constants of Rb and Si (Table 1) indicatesthat Rb is in principle a more optimal material as a spacer layer. Witha value of n at 13.4 nm similar to that of Si (close to unity), Rb wouldmaintain the optical contrast with e.g. Mo and Ru. In addition, thelower value of the extinction coefficient k compared to that of Si,makes Rb a near optimal spacer material. This is borne out by examples21 to 23 as can be seen from Table 2. An increase in the peakreflectivity of 5% is found for the Mo/Rb stack as compared to theequivalent Mo/Si stack yielding a value of R⁹int which is more than afactor 2 higher than the standard Mo/Si stack. However, Rb-based systemspresent constructional and operational difficulties due to the highreactivity and extremely low melting point (39° C.) of Rb.

REFERENCE EXAMPLE 24

Reference example 24 is a multilayer stack for use at 11.3 nm comprisingan unoptimised 80-period Mo/Be system grown on a Zerodur (RTM) glasssubstrate, with a partition ratio Γ=0.4 yielding d_(Mo)=2.3 nm andd_(Be)=3.4 nm. This provides the reference for examples 25 to 40 whichare tuned for use at 11.3 nm.

EXAMPLES 25 to 40

Table 3 corresponds to Table 2 but gives data for examples 25 to 40according to the invention which are reflector stacks tuned for use at11.3 nm.

The effects of optimization and the capping layer deposition are lessimportant at 11.3 nm than at 13.4 nm, only 8% improvement in R⁹int isprovided.

However, Ru and Rh are preferred to Mo for the 11.3 nm window. The Ru/Bestack has a relative optical throughput greater by up to 70% compared tothe Mo/Be reference example, whilst the throughput of the Rh/Be stack is33% greater. Although this is significantly lower than for Ru/Be, thiscombination may be preferable in some applications of the invention dueto factors such as Rh—Be interface chemistry.

A particularly preferred embodiment of the invention is the “needle”optimized Rh/Be stack which exhibits a huge increase in reflectivity.This is due to the incorporation of Pd, Ru and Mo layers during theoptimization process effectively transforming it into a Rh—Ru—Pd—Mo/Beor Pd—Rh—Ru—Mo/Be multi-component stack.

The layer thicknesses of an 80 period (80 Be layers) Ru—Sr/Be stackcapped with a 1.5 nm Ru layer are shown in FIG. 7. Similar results maybe achieved with Ru/Sr—Be. As before, the substrate surface is indicatedat layer 0. Due to their similar optical constants, Be and Sr performsimilar functions in the stack with Ru predominating near the substrate.The sum of the Be and Sr thicknesses near the surface is about 4.1 nmwhilst the Ru thickness is about 1.7 nm. These are markedly differentthan the thicknesses of the Mo/Be stack with Γ=0.4. This is because ofthe higher extinction coefficient of Ru, as compared to Mo, such that alower Ru thickness is preferred. The gain in employing Ru in place of Moderives from the resultant increase in optical contrast with Be. Thepreferred stack period is: Ru—Sr—Be.

Selected spectra of Be-based multilayers are shown in FIG. 8. ThisFigure shows plots of R⁹ vs. wavelength in the 11.3 nm region for fivestacks. A is the reference Mo/Be stack, B is an optimised Mo/Be stackwith a Ru capping layer, C is an optimised Ru/Be stack, D is a needleoptimised Rh/Be stack and E is an optimised, Ru-capped Ru—Sr/Be stack.

Examples 35 to 40 are strontium-containing three component systems whichyield throughput enhancements of up to a factor of 2.

As capping layers, Rh and Ru are optimum for this wavelength region andgive an increase of 0.7-1.0% in R.

EXAMPLES 41 to 44

From the above computational analysis of the various multilayer systemsfor the EUV region between 11 nm and 14 nm it would appear thatsignificant enhancements in peak reflectivities and the integratedreflectivities for a 9-mirror optical system are possible. A combinationof capping layer choice, global and needle optimisation routines and,most importantly, the incorporation of additional or replacementmaterials within the stack appears to be the recipe for reflectivityenhancement. Metals such as Rh and Ru which are generally easilydeposited using various vacuum deposition techniques provide advantages,especially in conjunction with Be for the 11.3 nm region where theysurpass Mo in theoretical performance. Furthermore, it is conceivablethat using the various combinations discussed above, problems ofinterface roughness associated with Mo/Si(Be) may be alleviatedsomewhat.

In for instance the Mo—Rh/Si and Mo—Ru/Si stacks, improved results areprovided with Rh(Ru) predominating over Mo near the substrate andvice-versa near the surface. This may be because at 13.4 nm Rh and Ruexhibit a higher optical contrast with Si than does Mo whereas theextinction coefficient k, and therefore the absorption within the layer,is lower for Mo than Rh and Ru. Near the surface of the stack, it isimportant that there be low absorption so that the incident radiationpenetrates as deep into the stack as possible so that the phasoraddition is maximized. However deep within the stack where the intensityis low, increased optical contrast is favored for the reflectedintensity to be maximized.

When Sr is incorporated in the structure it is preferentially located inthe near-surface region of the stack and partially substitutes Si. Thiscan be explained by similar arguments, the value of n for Sr is lowerthan that of Si and therefore while the optical contrast with the low-nmaterials is slightly lowered, the lower value of k for Sr compared withSi (see Table 1) means that the absorption within the layer is lowerthus favoring Sr near the surface of the stack. The data obtained forBe-based stacks for 11.3 nm operation indicates that similar effectsoccur.

Examples 41 to 44 are designed for use with a Xenon-jet laser-inducedplasma source (Xe-Jet LPS) which has a peak output intensity at about10.9 nm, somewhat lower than the range for which the reflectorsdescribed above were designed.

FIG. 9 shows the R⁹ reflectivities (left axis) of various reflectors andthe relative Xe-Jet LPS emission intensity (right axis) vs. wavelengthin nm (X axis). In FIG. 9:

(a) is the spectral response of the conventional unoptimized Mo/Si stackand is used as the reference for relative reflectivity figures.

(b) is an optimized Mo/Si stack similar to example 7 above;

(c) is an optimized Rh—Ru—Mo/Sr—Si stack;

(d) is a conventional, unoptimized, Mo/Be stack similar to comparativeexample 24 above;

(e) is an optimized Rh—Mo/Be stock similar to example 40 above;

(f) is an optimized Pd—Rh—Ru—Mo/Be stack;

(g) is an optimized Pd—Rh—Ru/RbCl stack forming example 41 of theinvention;

(h) is an optimized Rh—Ru/P stack forming example 42 of the invention;and

(i) is an optimized Rh—Ru/Sr stack forming example 43 of the invention.

Although examples 41 to 43 have lower R⁹ peak and R⁹int than otherexamples described above, they have the advantage of providing theirpeak reflectivity very close to the emission maximum of the Xe-Jet LPS.They are thus ideal for use with this source. Taking the throughput ofthe unoptimised Mo/Si stack as 1.0, examples 41(g), 42(h) and 43(i)provide relative throughputs of 3.0, 5.7, and 6.5 respectively. Thisalso compares well with the throughput of the Mo/Be stack (d), which is5.7 and avoids the use of Be, which is highly toxic.

Further improvements in peak reflectivity, giving values greater than0.75 in the 9.0 to 12 nm region can be attained in four component stacksthat combine P and Sr, e.g. Rh—Ru/P—Sr.

A further advance is shown by example 44. Example 44 is a needleoptimized Rh—Ru/Sr—Ce stack with a peak reflectivity of R=0.776 at 10.9nm. FIG. 10 shows the full wavelength dependence of R (left axis) and R⁹(right axis) of example 44 in the 10 to 12 nm range. FIG. 11 shows layerthicknesses in this stack.

EXAMPLES 45 to 48

Some further alternative stack configurations are shown in Table 4. Inthis table, Example 45 is a three layer stack of Ru—Nb/Si, whichdemonstrates that Niobium can also give improvements in an Si-basedstack, but is otherwise the same as the examples 8 to 12 of Table 2.

For use at 12.8 nm, different multilayers may be optimal. Two suchmultilayers are example 47 and 48 of Table 4. At 46, the R value of aconventional Mo/Si (equivalent to Comparative Example 1) at 12.8 nm isgiven. It can readily be seen that the addition of Ru partiallyreplacing Mo improves reflectivity at this frequency while the use ofberyllium as a spacer material partially replacing silicon providesfurther improvements.

In general, the lanthanides (rare earth metals) may provide good opticalcontrast with metals such as Mo, Ru and Rh and may be preferred inreflectors nearer the substrate. In this position, optical contrast isprovided because the lanthanides have a refractive index n very close tounity which outweighs the disadvantage that their values of extinctioncoefficient k are not as low as some other materials in the 9-16 nmregion. Lanthanum is particularly preferred at or near 13 nm.

Further alternative spacers useable in the invention are porousmaterials such as low density (porous) silica (aerogel) having a densityabout 1 tenth that of bulk silica. FIG. 12 shows the wavelengthsensitivity of a Rh—Ru/SiO₂-aero stack using such porous silica. Itsrelatively broad reflectance peak below 11 nm will be noted. Other lowdensity materials that may be used include: titania and aluminaaerogels; nano-porous silicon, meso-porous silicon, nanoclusters ofsilicon and other semiconductors. These materials may be used tomanufacture reflectors tuned to specific wavelengths throughout the 8 to20 nm wavelength range. The materials are useful because the values on nand k are density dependent. With decreasing density the refractiveindex, n, tends to unity and the extinction coefficient, k, tends tozero. The density of a typical Si aerogel is 0.2 gcm⁻³ whilst that ofporous Si is 1.63 gcm⁻³.

EXAMPLES 49 to 65

Further examples of useful capping layers are set out in Tables 5 and 6,which give the same data as previous tables.

In Table 5, 49 is a comparative example consisting of an optimized (for13.4 nm) 50 period Mo/Si stack whose outermost layer is 2 nm of SiO₂formed by natural oxidation of the final Si layer in the stack. Thiscomparative example forms the reference for relative values of R⁹peakand R⁹int for Examples 50 to 57 of the invention. These examples differfrom comparative example 49 only in the indicated capping layer, whichis deposited on final Si layer of the stack before that layer canoxidized. It will be seen that each of palladium (Pd), boron carbide(B₄C), boron nitride (BN), silicon carbide (SiC), silicon nitride(Si₃N₄) and diamond-like carbon (dl-C) exhibit improved reflectance, oran acceptable reduction, whilst exhibiting a high degree of resistanceto chamical attack.

In Table 6, 58 is a comparative example consisting of an 80 periodoptimized (for 11.3 nm) Mo/Be stack, similarly with an outermost layerof 2 nm BeO formed by natural oxidation of the final Be layer. Thiscomparative example forms the reference for the relative values ofR⁹peak and R⁹int for Examples 59 to 65 of the invention. Examples 59 to65 differ from comparative example 58 in the indicated capping layerwhich is deposited before the outer Be layer can oxidized. It will againbe seen that the layers specified provide improved reflectivity, or anacceptable reduction, whilst exhibiting a high degree of resistance tochemical attack.

EXAMPLES 66 to 76

In examples 66 to 76 the capping layer includes a modified final layerof the multilayer coating as well as a dedicated capping sublayer so asto form a bi- or tri-layer protective structure thus increasing theoverall thickness of the top layers and reducing the likelihood ofincomplete coverage through multiple layer deposition. This isillustrated in FIG. 13.

The reflector of examples 66 to 76 of the invention comprises substrate10 on which are deposited N periods of alternating layers of a firstmaterial 11 and a second material 12. In FIG. 13 only the first period13 is shown however all periods save the last are similar. The final,N^(th) period comprises a layer 15 of the first material, a layer 16 ofa third material and a capping sub-layer 17 of a capping material. Inthe following, the first material is denoted X, the second material Yand the third material Z.

The first material X is one or more of: Mo, Ru, Rh, Nb, Pd, Y and Zr,and the second material Y is one or more of: Be, Si, Sr, Rb, RbCl and P.The final period is constructed such that the substance X is chosen aspreviously, the third material Z on the other hand, is chosen from a setof materials with a moderately high value of refractive index n (>0.96),sufficiently low value of the extinction coefficient k (<0.01), andwhich are known for their chemical inertness and stability. For the10-15 nm spectral region the following materials are suitable: B₄C, BN,diamond-like C, Si₃N₄ and SiC. Although these materials are not ideal“spacers”, the reflectivity loss through absorption in layer 16 may betolerated in favour of long-term chemical and structural integrity ofthe multilayer. In addition, the combination of layers 15 and 16 has atotal optical thickness of ^(˜)2 quarter wavelengths (where thequarter-wave optical thickness is given by: QW=4 nd/λ), thuscontributing to the reflection coefficient and avoiding a drasticreduction in the reflectivity which may be caused by relatively thick(>3 nm) capping layers. In addition the material of the capping layer 17has low n such that a large optical contrast is maintained betweenlayers 16 and 17. The boundary between layers 16 and 17 also serves tolocalise the node of the standing wave formed through the superpositionof the incident and reflected waves. Suitable materials for cappinglayer 17 in this configuration are: Ru, Rh, Pd and diamond-like C.

Table 7 shows layer materials and thicknesses for Examples 66 to 71which comprise 79 periods of Mo/Be plus the additional period X/Zconstructed as described above. These examples are intended for use at11.3 nm. In example 66, the whole of the Be layer is oxidized and a Rucapping layer is deposited. This is the reference example. Example 67shows that SiC is not ideal for the 11.3 nm region. However, Examples 70and 71 show clearly that values of R greater than 75.5% are stillpossible with such a configuration. Rh is used to replace the Mo layeron account of its inertness and C or B₄C is deposited as layer 16 withan additional coating of Ru as layer 17. This gives a tri-layer ofthickness of 7.7 nm forming the protective coating structure. Examples68 and 69 are analogous to 70 and 71 respectively, with the importantdistinction that the thickness of the layer 17 is increased by 2QWresulting in lower, but still respectable, reflectivity values and witha substantially higher tri-layer thickness of 13.7 nm.

Similarly, Table 8 shows layer materials and thicknesses for Examples 72to 76 which comprise 49 periods of Mo/Si with the additional periodformed by the X/Z combination again terminated with a Ru capping layer.The reference example 72 represents a fully oxidised top Si layer uponwhich a Ru capping layer is applied. SiC and B₄C are the most favorablematerials for the Z layer 16. However, at 13.4, for which these examplesare intended, Mo cannot be replaced by the more inert metal Rh,therefore a bi-layer protective structure is formed where the combinedthickness of layers 16 and 17 (d_(Z)+d_(CL)) is about 5.5-6.0 nm. Inexample 73 the thickness of the SiC layer is increased by 2QW resultingin a 12.6 nm protective bi-layer thickness at the expense ofreflectivity.

Other suitable materials for the capping layer are Au, MgF₂, LiF, C2F₄(teflon) and TiN

While we have described above specific embodiments of the invention itwill be appreciated that the invention may be practiced otherwise thanas described. The description is not intended to limit the invention.

TABLE 1 10.9 nm 11.3 nm 13.4 nm n k n k n k B 0.9786 0.0023 0.96890.0040 B₄C 0.9753 0.0029 0.9643 0.0050 Be 1.0092 0.0196 1.0081 0.00100.9892 0.0018 BeO 0.9785 0.0102 0.9587 0.0171 BN 0.9740 0.0050 0.96330.0086 C 0.9732 0.0040 0.9622 0.0067 Ce 1.0522 0.0197 1.0380 0.01591.0074 0.0062 Eu 0.9902 0.0062 0.9883 0.0074 0.9812 0.0123 La 1.07770.0601 1.0460 0.0200 1.0050 0.0065 Mo 0.9514 0.0046 0.9227 0.0062 P0.9949 0.0014 Pd 0.9277 0.0099 0.9198 0.0135 0.8780 0.0443 Pr 1.01670.0119 1.0115 0.0125 0.9840 0.0072 Rb 0.9974 0.0014 0.9941 0.0007 RbCl0.9943 0.0023 0.9941 0.0022 0.9895 0.0019 Rh 0.9313 0.0068 0.9236 0.00890.8775 0.0296 Ru 0.9373 0.0056 0.9308 0.0063 0.8898 0.0165 Si 1.00550.0146 0.9999 0.0018 Si aerogel 0.9988 0.0011 Porous Si 1.0015 0.0049Si₃N₄ 0.9864 0.0173 0.9741 0.0092 SiC 0.9936 0.0159 0.9831 0.0047 SiO₂0.9865 0.0123 0.9787 0.0106 Sr 0.9936 0.0011 0.9928 0.0011 0.9880 0.0013Y 0.9835 0.0020 0.9742 0.0023 Zr 0.9733 0.0029 0.9585 0.0037

TABLE 2 R R⁹peak R⁹int 1 Mo/Si N 2 nm SiO₂ 0.731 1.00 1.00 2 Mo/Si N (2nm Si +) 0.741 1.13 1.07 2 nm SiO₂ 3 Mo/Si N 2 nm B 0.751 1.27 1.25 4Mo/Si Y 2 nm B 0.752 1.29 1.26 5 Mo/Si Y 1.5 nm Rh 0.754 1.32 1.27 6Mo/Si N 1.5 nm Ru 0.757 1.37 1.35 7 Mo/Si Y 1.7 nm Ru 0.758 1.39 1.36 8Mo-Rh/Si Y 1.7 nm Ru 0.762 1.45 1.38 9 Mo-RbCl/Si Y 1.5 nm Ru 0.761 1.441.39 10 Mo-Ru/Si Y 1.5 nm Rh 0.760 1.42 1.41 11 Mo-Ru/Si Y 1.7 nm Ru0.765 1.51 1.50 12 Mo-Ru/Si Y(n) 1.5 nm Ru 0.764 1.48 1.59 13Mo-Rh-RbCl/Si Y 1.7 nm Ru 0.764 1.49 1.38 14 Mo-Ru-Zr/Si Y 1.7 nm Ru0.764 1.49 1.44 15 Mo-Ru-Y/Si Y 1.5 nm Ru 0.770 1.60 1.55 16Mo-Ru-RbCl/Si Y 1.5 nm Ru 0.767 1.54 1.56 17 Mo-Rh-Sr/Si Y 1.6 nm Ru0.779 1.77 1.56 18 Mo-Ru-Sr/Si Y 1.5 nm Rh 0.776 1.71 1.57 19Mo-Ru-Sr/Si Y 1.5 nm Ru 0.791 1.81 1.68 20 Mo-Ru-Sr/Si Y(n) 1.5 nm Ru0.781 1.81 1.85 21 Ru/Rb Y 1.5 nm Ru 0.779 1.77 1.41 22 Mo/Rb Y 1.5 nmRu 0.809 2.49 2.13 23 Mo-Ru-Sr/Rb Y 1.5 nm Ru 0.814 2.63 2.20

TABLE 3 R R⁹peak R⁹int 24 Mo/Be N 2 None 0.775 1.00 1.00 25 Mo/Be N 1.5nm Rh 0.782 1.08 1.08 26 Mo/Be Y None 0.780 1.06 1.00 27 Mo/Be Y 1.5 nmRh 0.787 1.15 1.06 28 Mo/Be Y 1.5 nm Ru 0.788 1.16 1.08 29 Ru/Be Y 1.5nm Rh 0.810 1.49 1.68 30 Ru/Be Y 1.5 nm Ru 0.811 1.50 1.70 31 Rh/Be N1.5 nm Rh 0.793 1.10 1.33 32 Rh/Be Y 1.5 nm Rh 0.793 1.23 1.29 33 Rh/BeY 1.5 nm Ru 0.794 1.24 1.31 34 Rh/Be Y(n) 1.5 nm Rh 0.811 1.50 1.77 35Mo-Sr/Be Y 1.5 nm Rh 0.799 1.32 1.21 36 Ru-Sr/Be Y 1.5 nm Rh 0.822 1.701.97 37 Ru-Sr/Be Y 1.5 nm Ru 0.823 1.72 2.00 38 Rh-Sr/Be Y 1.5 nm Rh0.810 1.49 1.64 39 Rh-Sr/Be Y 1.5 nm Ru 0.811 1.50 1.67 40 Ru-Mo/Be Y(n)1.5 nm Ru 0.812 1.52 1.72

TABLE 4 R R⁹peak R⁹int 45 Ru-Nb/Si Y 2 nm Rh 0.754 1.20 1.27 46 Mo/Si N2 nm Si + 2 nm 0.738 1.00 1.00 SiO₂ 47 Ru-Mo/Si Y 2 nm Rh 0.768 1.431.48 48 Ru-Mo/Be-Si Y 2 nm Rh 0.778 1.61 1.63

TABLE 5 R R⁹peak R⁹int 49 Mo/Si Y 2 nm SiO₂ 0.745 1.00 1.00 50 Mo/Si Y 2nm Pd 0.743 0.97 0.92 51 Mo/Si Y 2 nm Si₃N₄ 0.747 1.01 1.02 52 Mo/Si Y 2nm SiC 0.748 1.03 1.04 53 Mo/Si Y 2 nm BN 0.749 1.04 1.05 54 Mo/Si Y 2nm Rh 0.751 1.06 1.05 55 Mo/Si Y 2 nm (dl-)C 0.750 1.06 1.08 56 Mo/Si Y2 nm B₄C 0.751 1.07 1.10 57 Mo/Si Y 2 nm Ru 0.758 1.16 1.17

TABLE 6 R R⁹peak R⁹int 58 Mo/Be Y 2 nm BeO 0.774 1.00 1.00 59 Mo/Be Y 2nm SiC 0.769 0.94 0.92 60 Mo/Be Y 2 nm BN 0.779 1.06 1.09 61 Mo/Be Y 2nm Pd 0.781 1.09 1.10 62 Mo/Be Y 2 nm (dl-)C 0.781 1.08 1.11 63 Mo/Be Y2 nm B₄C 0.782 1.09 1.13 64 Mo/Be Y 2 nm Rh 0.786 1.15 1.18 65 Mo/Be Y 2nm Ru 0.788 1.17 1.21

TABLE 7 X/Y X Z CL R R⁹peak R⁹int 66 Mo/Be 2.05 nm 3.77 nm 2.03 nm 0.7171.00 1.00 (0.69 QW) (1.31 QW) Ru Mo BeO 67 Mo/Be 4.12 nm 1.93 nm 2.04 nm0.713 0.95 0.91 (1.35 QW) (0.68 QW) Ru Rh SiC 68 Mo/Be 1.70 nm 9.95 nm2.03 nm 0.721 1.05 1.09 (0.56 QW) (3.43 QW) Ru Rh C 69 Mo/Be 1.56 nm10.06 nm 1.96 nm 0.739 1.30 1.25 (0.51 QW) (3.47 QW) Ru Rh B₄C 70 Mo/Be1.70 nm 4.15 nm 1.90 nm 0.756 1.61 1.57 (0.56 QW) (1.43 QW) Ru Rh C 71Mo/Be 1.56 nm 4.27 nm 1.85 nm 0.765 1.78 1.73 (0.51 QW) (1.47 QW) Ru RhB₄C

TABLE 8 X/Y X Z CL R R⁹peak R⁹int 72 Mo/Si 2.84 nm 4.24 nm 2.05 nm 0.6991.00 1.00 (0.78 QW) (1.24 QW) Ru Mo SiO₂ 73 Mo/Si 3.28 nm 10.63 nm 2.06nm 0.696 0.97 0.93 (0.90 QW) (3.12 QW) Ru Mo SiC 74 Mo/Si 3.87 nm 3.38nm 1.97 nm 0.716 1.24 1.21 (1.07 QW) (0.97 QW) Ru Mo C 75 Mo/Si 3.23 nm3.95 nm 1.92 nm 0.725 1.39 1.36 (0.89 QW) (1.14 QW) Ru Mo B₄C 76 Mo/Si3.28 nm 3.52 nm 1.87 nm 0.735 1.57 1.53 (0.90 QW) (1.12 QW) Ru Mo SiC

We claim:
 1. A lithographic projection apparatus, comprising: anillumination system constructed and arranged to supply a projection beamof radiation; a first object table provided with a first object holderconstructed and arranged to hold a mask; a second object table providedwith a second object holder constructed and arranged to hold asubstrate; a projection system constructed and arranged to utilize saidradiation to image an irradiated portion of the mask onto a targetportion of the substrate; and at least one of said illumination systemand projection system having an optical element with a surface on whichradiation is incident and a capping layer covering said surface, saidcapping layer being formed of a relatively inert material, wherein saidrelatively inert material is selected from the group consisting of:diamond-like carbon, Ru, Rh, TiN, MgF₂, LiF, C₂F₄ and compounds andalloys thereof.
 2. Apparatus according to claim 1 wherein saidrelatively inert material is more inert than material from whichremaining portions of said optical element are formed.
 3. Apparatusaccording to claim 1 wherein said relatively inert material is lesseasily oxidized than the material from which remaining portions of saidoptical element are formed.
 4. Apparatus according to claim 1, whereinsaid relatively inert material is harder than material from whichremaining portions of said optical element is formed.
 5. Apparatusaccording to claim 1 wherein said optical element is a beam modifyingelement.
 6. Apparatus according to claim 5 wherein said optical elementis a reflector having a multilayer coating on which said capping layeris provided.
 7. Apparatus according to claim 1 wherein said opticalelement is a sensor.
 8. Apparatus according to claim 1 wherein saidcapping layer comprises two sub-layers of different materials. 9.Apparatus according to claim 1 wherein said projection beam comprisesradiation, having a wavelength in the range of from 8 nm to 20 nm. 10.Apparatus according to claim 9 wherein said projection beam comprisesradiation having a wavelength in the range of from 9 nm to 16 nm. 11.Apparatus according to any one of the preceding claims wherein saidcapping layer has a thickness in the range of from 0.5 nm to 10 nm. 12.Apparatus according to claim 11 wherein said capping layer has athickness in the range of from 0.5 nm to 6 nm.
 13. Apparatus accordingto claim 12 wherein said capping layer has a thickness in the range offrom 0.5 nm to 3 nm.
 14. A device manufacturing method using alithographic apparatus, the method comprising: providing a maskcontaining a pattern to a first object table; providing a substrate atleast partially covered by a layer of energy-sensitive material to asecond object table; and irradiating said mask and imaging irradiatedportions of said pattern onto said substrate; said irradiatingcomprising directing radiation onto a surface of an optical element, thesurface having a capping layer formed of a relatively inert material,wherein said relatively inert material is selected from the groupconsisting of: diamond-like carbon, Ru, Rh, TiN, MgF₂, LiF, C₂F₄ andcompounds and alloys thereof.
 15. A semiconductor device manufactured inaccordance with the method of claim
 14. 16. A lithographic projectionapparatus, comprising: an illumination system constructed and arrangedto supply a projection beam of radiation; a first object table providedwith a first object holder constructed and arranged to hold a mask; asecond object table provided with a second object holder constructed andarranged to hold a substrate; a projection system constructed andarranged to utilize said radiation to image an irradiated portion of themask onto a target portion of the substrate; and at least one of saidillumination system and projection system having a sensor with a surfaceon which radiation is incident and a capping layer covering saidsurface, said capping layer being formed of a relatively inert material,wherein said relatively inert material is selected from the groupconsisting of: diamond-like carbon (C), Ru, Rh, Au, MgF₂, LiF, C₂F₄, TiNand compounds and alloys thereof.
 17. The lithographic projectionapparatus according to claim 16, wherein said relatively inert materialis more inert than material from which remaining portions of said sensorare formed.
 18. The lithographic projection apparatus according to claim16, wherein said relatively inert material is less easily oxidized thanthe material from which remaining portions of said sensor are formed.19. The lithographic projection apparatus according to claim 16, whereinsaid relatively inert material is harder than material from whichremaining portions of said sensor is formed.
 20. The lithographicprojection apparatus according to claim 16, wherein said capping layerhas a thickness in the range of from 0.5 nm to 10 nm.
 21. Thelithographic projection apparatus according to claim 20, wherein saidcapping layer has a thickness in the range of from 0.5 nm to 6 nm. 22.The lithographic projection apparatus according to claim 20, whereinsaid capping layer has a thickness in the range of from 0.5 nm to 3 nm.23. The lithographic projection apparatus according to claim 16, whereinsaid capping layer comprises two sub-layers of different materials. 24.The lithographic projection apparatus according to claim 16, whereinsaid projection beam comprises radiation having a wavelength in therange of from 8 nm to 20 nm.
 25. A lithographic projection apparatus,comprising: an illumination system constructed an arranged to supply aprojection beam of radiation; a first object table provided with a firstobject holder constructed and arranged to hold a mask; a second objecttable provided with a second object holder constructed and arranged tohold a substrate; a projection system constructed and arranged toutilize said radiation to image an irradiated portion of the mask onto atarget portion of the substrate; and at least one of said illuminationsystem and projection system having an optical element with a surface onwhich radiation is incident and a capping layer covering said surface,said capping layer being formed of a relative inert material, whereinsaid optical element is a reflector having a multilayer reflectivecoating on which said capping layer is provided; and wherein saidrelative inert material is selected from the group consisting of:diamond-like carbon (C), boron-nitride (BN), boron carbide (B₄C),silicon nitride (Si₃N₄), silicon carbide (SiC), B, Pd, Ru, Rh, Au, MgF₂,LiF, C₂F₄, TiN and compounds and alloys thereof.
 26. The lithographicprojection apparatus according to claim 25, wherein said multilayerreflective coating comprises a plurality of layers of a first materialhaving a relatively low reflective index at the wavelength of saidprojection beam.
 27. The lithographic projection apparatus according toclaim 26, wherein said multilayer reflective coating further comprises aplurality of layers of a second material having a relatively highreflective index at the wavelength and alternating with said layers ofsaid first material.
 28. The lithographic projection apparatus accordingto claim 25, wherein said relatively inert material is more inert thanmaterial from which remaining portions of said optical element areformed.
 29. The lithographic projection apparatus according to claim 25,wherein said relatively inert material is less easily oxidized than thematerial from which remaining portions of said optical element areformed.
 30. The lithographic projection apparatus according to claim 25,wherein said relatively inert material is harder than material fromwhich remaining portions of said optical element is formed.
 31. Thelithographic projection apparatus according to claim 25, wherein saidcapping layer has a thickness in the range of from 0.5 nm to 10 nm. 32.The lithographic projection apparatus according to claim 31, whereinsaid capping layer has a thickness in the range of from 0.5 nm to 6 nm.33. The lithographic projection apparatus according to claim 31, whereinsaid capping layer has a thickness in the range of from 0.5 nm to 3 nm.34. The lithographic projection apparatus according to claim 25, whereinsaid projection beam comprises radiation having a wavelength in therange of from 8 nm to 20 nm.
 35. A lithographic projection apparatus,comprising: an illumination system constructed and arranged to supply aprojection beam of radiation; a first object table provided with a firstobject holder constructed and arranged to hold a mask; a second objecttable provided with a second object holder constructed and arranged tohold a substrate; a projection system constructed and arranged toutilize said radiation to image an irradiated portion of the mask onto atarget portion of the substrate; and at least one of said illuminationsystem and projection system having an optical element with a surface onwhich radiation is incident and a capping layer covering said surface,said capping layer being formed of a relatively inert material, whereinsaid optical element comprises: a reflector having a multilayerreflective coating on said surface, said multilayer reflective coatingcomprising a plurality of layers of a first material having a relativelylow refractive index at the wavelength of said projection beam; layersof a second material having a relatively high refractive index at saidwavelength and alternating with said layers of said first material; andsaid capping layer comprises: a first sub-layer of said first material;a second sub-layer of a third material having a refractive index at saidwavelength higher than said first material and being more inert thansaid second material; and a third sub-layer formed of a fourth materialthat is relatively inert, said first, second and third sub-layers beingprovided in that order with said third sub-layer outermost.
 36. Thelithographic projection apparatus according to claim 35, wherein saidthird material has a refractive index at said wavelength greater thanabout 0.95 and an extinction coefficient at said wavelength less thanabout 0.01.
 37. The lithographic projection apparatus according to claim36, wherein said first material is one or more materials selected fromthe group consisting of Mo, Ru, Rh, Nb, Pd, Y and Zr, as well ascompounds and alloys of these elements; said second material is one ormore materials selected from the group consisting of Be, Si, Sr, Rb,RbCl and P, as well as compounds and alloys thereof; said third materialis selected from the group consisting of B₄C, BN, diamond-like carbon(C), Si₃N₄ and SiC; and said fourth material is selected from the groupconsisting of Au, Ru, Rh, Pd, B, MgF₂, LiF, C₂F₄, TiN, boron nitride(BN), boron carbide (B₄C₉), silicon nitride (Si₃N₄), Silicon carbide(SiC), diamond-like carbon (C), and compounds and alloys thereof. 38.The lithographic projection apparatus according to claim 35, whereinsaid projection beam comprises radiation having a wavelength in therange of from 8 nm to 20nm.